Taiji learners like to talk about the five bows in the body. It is said that "Fa Jing" is like shooting out an arrow.
However, very few know exactly how the bows operate. Without knowing how the bows operate, one can never shoot out an arrow successfully, not to say with power.
The most important member of the five bows is the body bow. Many contemporary taiji commentators say that the body bow is the spine. According to them, what you need to do is to curve your spine and make it straight again, just like pulling the string of the bow and release it, in order to send out the power.
It gets you nowhere if your follow this method.
It is true that you need to move your spine in order to send out the internal force. However, it cannot be done by curving and straightening your spine in the normal way. You need to activate your back muscles and the related power channels in the very first place in order to utilise the power of your body bow. Before your back muscles are activated and the related power channels opened, the purported curving and straightening of your spine is meaningless - You only curve / straighten your body by contracting / extending the muscles in the front part of your body. Your spine is still sleeping.
You need to think about "fa jing" from the perspective of exercising coherent bodily force. It is not just a matter of opening the bows (all five bows). It is the coordination of the whole body, whereupon the whole body contracts or extends coherently and in a very short slice of time, with your back performing the decisive role.
Assuming that you have activated your back muscles and opened the related power channels. The next step is to find out the way to pull the string of the bow. The method is to contract the back muscles and then extend it. It is the converse of opening the string of a physical bow where you extend the string first and then release it to let it contract again.
It is the expansion of power that sends the power arrow out. You cannot send out the arrow by the other way round.